[Attila85]

Yes I did but still no results.
Here is the code I added in the header template:

<!-- Display Online Users Count -->
<b>{vb:raw msxtotal}</b> utenti online ({vb:rawphrase x_members_and_y_guests, {vb:raw msxmembers}, {vb:raw msxguests}})
<!-- / Display Online Users Count -->

Here the code of the plugin:

// get online users 
$datecut = TIMENOW - $vbulletin->options['cookietimeout']; 
$guestsarry = $db->query_first("SELECT COUNT(host) AS sessions FROM " . TABLE_PREFIX . "session WHERE userid = 0 AND lastactivity > $datecut");
$membersarry = $db->query_read("SELECT DISTINCT userid FROM " . TABLE_PREFIX . "session WHERE userid <> 0 AND lastactivity > $datecut"); 
$msxguests = intval($guestsarry['sessions']); 
$msxmembers = intval($db->num_rows($membersarry)); 
$msxtotal = $msxmembers + $msxguests; 
vB_Template::preRegister('header',array('msxtotal' => $msxtotal, 'msxmembers' => $msxmembers, 'msxguests' => $msxguests)); 
// get online users

Position: Global_start
Execution order: 5
Plugin is active.

But still no results, as you can see... any hint?